This invention relates to alkaline secondary electrochemical batteries which utilize zinc as the active electrode material and, more particularly, it relates to the use of mercury-free zinc electrodes in such cells and to improved negative interseparators for use in such cells.
Zinc is used as the negative electrode material in alkaline electrochemical cells and batteries for a number of reasons including its high half-cell voltage, its low polarization, and its high limiting current density on discharge. However, a number of adverse effects result from such use of zinc. Thus, because it is thermodynamically unstable in alkaline media, zinc self-discharges significantly while standing. Such discharge is accompanied by the evolution of hydrogen. Together, these reactions produce electrode corrosion and shortened shelf life.
In order to overcome the aforementioned disadvantages while still taking advantage of the benefits derived from the use of zinc, battery manufacturers normally include small amounts of mercury in zinc electrodes to reduce self-discharging of the latter and to reduce evolution of hydrogen. Such reductions are made possible by the fact that mercury has a substantially higher hydrogen overvoltage than does zinc.
Although the use of mercury in zinc electrodes is advantageous, its presence in such cells and batteries and the processes associated with the introduction of mercury into such cells and batteries present both environmental and health hazards. Therefore, it would be desirable to be able to produce mercury-free zinc electrodes having electrochemical characteristics similar to or better than mercury-containing zinc electrodes.
Inorganic additives other than mercury have been included in zinc electrodes for some time in order to reduce the corrosion of such electrodes by reducing their self-discharge in alkaline solution. Suggested additives have included: lead, lead oxide, lead hydroxide, lead sulfide, indium hydroxide, stannous chloride and thallium oxide. The aforementioned additives may be used in the absence of or in combination with mercury in zinc electrodes. Exemplary of the use of such additives in zinc electrodes are U.S. Pat. Nos. 3,639,176, 3,642,539, 3,847,669, 3,816,178 and 3,785,868. Although these additives have been found to be partially effective in reducing self-discharge of zinc electrodes in alkaline media, the same additives may adversely affect the potential-current curve on discharge, particularly at high battery drain rates.